1. Field of the Invention
The present invention relates to a switching system, more particularly to a system including therein a tone trunk (TNT) which produces a variety of digital tone signals to be supplied, after conversion into analog tone signals, to subscriber's telephone sets.
2. Description of the Related Art
Generally, there are two methods of producing each tone signal from the tone trunk to the subscriber's telephone set. First, the tone trunk produces an analog tone signal and supplies it to the subscriber as is. Second, the tone trunk stores, in advance, a tone in the form of a digital signal, i.e., a digital tone signal, in its digital memory, and the digital tone signal is read therefrom. And then, when required, the digital tone signal supplied, after conversion into the corresponding analog tone signal, to the related subscriber. Of the above-mentioned two methods, the switching system of the present invention uses the tone signals produced as in the latter method, i.e., digitally processed tone signals.
The thus produced digital tone signals are supplied from the tone trunk to the subscribers in the form of a dial tone (DT), a ring-back tone (RBT), a busy tone (BT), a reorder tone (ROT), a hold tone (HT), and the like, selectively. These digital tone signals are stored in the digital memory of the tone trunk, such as a read-only memory (ROM).
Usually, each digital tone signal is obtained by first sampling a corresponding analog tone signal and then coding the thus-sampled analog tone signal with a predetermined coding rule, such as a known pulse code modulation (PCM) coding rule. In the usual PCM coding rule, each sampling analog tone signal level is quantized into a corresponding quantization level. Among the many different quantization levels, the quantization level is selected by determining a level which is closest to the sampling analog tone signal level.
It is commonly known that the thus-obtained digital tone signal is accompanied, when converted into the audible analog tone signal, by a quantizing noise, and from general experience, such a quantizing noise is inoffensive to the listener when the noise accompanies a random analog tone signal such as a voice signal.
The problem in the prior art switching system having the digital tone trunk is that the quantizing noise is offensive to the listener when the noise accompanies a monotonous tone such as the dial tone (DT), the ring-back tone (RBT) and the like. Thus, the quantizing noise is not negligible with regard to the dial tone and so on, and is heard as a single tone superimposed on the analog tone to be communicated. Although the level of the single tone is considerably low, it is offensive to the ears of the subscribers.